This invention relates to a thermally responsive switch suitable for protecting electric motors employed in hermetic compressors of refrigerating machines against burnout due to overheating and a method of making the same.
The motor employed in the hermetic compressor for the refrigerating machine is usually driven in a hermetically sealed compressor housing with refrigerant and lubricating oil surrounding it. Taking into consideration the maximum pressure values in low and high pressure conditions during respective compressor on and off periods, the pressure in the hermetically sealed compressor housing is varied in a vast range. The thermally responsive switch used in the above-described atmosphere is required to be reliably responsive to changes in the motor winding temperature and the current and to open an electrical path in an abnormal condition so that the motor is deenergized. In order to operate the thermally responsive switch as described above, its parts including movable and fixed contacts are enclosed in a hermetic casing so that invasion of the refrigerant or the like into the casing interior can be prevented. Furthermore, the hermetic casing of the thermally responsive switch necessitates a high level of pressure tightness, thermal responsiveness and specific characteristic of distinguishing between a normal current and an abnormal current. Additionally, the thermally responsive switch is required to be small in size, large in the switching capacity and superior in durability while it should be stable in quality and cost effective. Under these circumstances, it has become difficult to provide a thermally responsive switch meeting the above-described demands.
Conventional thermally responsive switches are disclosed in Japanese Published Patent Application (kokoku) Nos. 58-56213 and 62-6294. As obvious from the foregoing, the hermetic casing of the thermally responsive switch needs to have a sufficient pressure tightness so that the responsiveness of the device is not affected by severe temperature and pressure changes in the compressor housing. In these conventional devices, however, the increase in the pressure tightness extremely increases the wall thickness of the hermetic casing enclosing the switching assembly including a bimetallic thermally responsive element carrying a movable contact and a fixed contact engaged with and disengaged from the movable contact, resulting in a disadvantage. Furthermore, the position of the movable contact relative to the fixed contact is checked by means of X-ray irradiation after the switch receptacle is finally sealed hermetically. Thus, checking the position of the movable contact relative to the fixed contact cannot be readily performed. Additionally, the thermally responsive switch repeatedly break a motor circuit in response to an abnormal current flowing into the motor or an overtemperature and make the motor circuit when a normal current condition or a safe temperature condition is recovered. The service life of the thermally responsive switch depends largely upon the number of circuit making and breaking operations of the contacts. Heat due to an arc between the contacts melts the contact surface. Particles of melted contact material are caused to splash around to adhere to the surface of an electrical insulator insulating a portion at the same potential as a movable contact and a portion at the same potential as a fixed contact, resulting in gradual decrease in an insulation distance between the fixed and movable contacts. This decrease in the insulation distance reduces a dielectric strength, which reduction in the dielectric strength is one of important causes of shortening the life of the thermally responsive switch. Although this may be solved by increasing the insulation distance between the contacts, the increase in the insulation distance increases the dimensions of the thermally responsive switch. Furthermore, the insulator itself is required to have such a particular heat resistance that they are not broken by the heat due to the arc between the contacts.